This invention relates to an antenna and more particularly to one operable for transmitting and receiving electromagnetic radiation at frequencies above 30 mHz using reflecting surfaces.
Communication antennas for ground stations used in links with satellites in geostationary orbits are required by the Federal Communications Commission and the International Radio Consultative Committee to have sidelobe levels outside an angle .theta.=1.degree. cone about their main beams below the level of EQU 32-25 log.sub.10 .theta.
in decibels referred to an isotropic radiator and an axial ratio for circular polarization that does not exceed 1.09. These stringent specifications for sidelobe levels and polarization purity are not met by many antennas currently installed. It is economically important that aperture efficiencies on large reflector antennas used in satellite communications be as high as possible in order to realize high antenna gains with smallest possible reflector areas.
Most present day ground station antennas for satellite links are reflector antennas fed by Cassegrain subreflectors and horns symmetrically located on the reflector axis such that subreflector and horn are directly in front of the main reflector (U.S. Pat. Nos. 4,044,361, 3,983,560, 3,995,275, 3,821,746, 3,562,753). This configuration of the reflector feed causes aperture blocking which, in turn, produces unwanted sidelobes generally in the direction of communication satellites located about 35,800 kilometers above the earth's surface in orbits about the earth's equator. These ground-based reflector antennas are generally mounted on a pedestal which moves the entire antenna in the direction of a satellite for tracking slight relative angular motions of the satellite which is emitting signals to, or receiving signals from, the antenna. Large reflector antennas mounted on a pedestal are subject to reflector surface deformation due to gravitational and wind loading. The struts in front of the reflector aperture used to support the horn and subreflector also cause increase in sidelobe levels. The in-line arrangement of horn, subreflector and main reflector causes specular reflections back to the horn which produces an unwanted increase in voltage standing wave ratios. Electromagnetic energy is lost due to spillover which means not all radiation from the horn separated from the subreflector strikes the subreflector, and not all radiation from the subreflector strikes the main reflector. When the subreflector surface is enlarged to give a sharper pattern gradient at the edge of the main reflector, the blocking sidelobes levels increase. Offset feeding (U.S. Pat. Nos. 3,914,768; 3,949,404; 3,810,187; 3,332,083; 3,500,427; 3,936,837; 3,792,480) has been used to improve the performance of antennas for radar and satellite communications. However, the aperture efficiency for prior art antennas has been low because no means was known for shaping the asymmetrically located subreflectors to produce the nearly uniform aperture illumination which is needed for high aperture efficiency. Antenna beam scanning by feed motion is known. (See U.S. Pat. Nos. 3,500,427; 3,914,768; 3,641,577; 3,745,582). However, no means for fully correcting optical aberrations, which cause aperture phase errors contributing to increased sidelobe levels and loss in antenna gain on offset fed reflectors, has been reported when the antenna beams are pointed away from the principal axis of the main reflector. Furthermore, no means is known for correcting optical aberrations on feed systems using shaped subreflectors and horns scanned or producing more than one beam by feed motion or displacement from a preferred orientation.
With reference to prior art, there are three patents which, although they relate to the objectives of the present invention, differ in fundamental aspects from the antenna system to be described. The invention of Bartlett and Sheppard, U.S. Pat. No. 3,737,909, improved the antenna aperture illumination efficiency by use of a dielectric refractive element. This technology is restricted to antennas with rotational symmetry about the main reflector axis and not applied to offset geometry. The method for design uses conventional integral relations between the feed power angular distributions and the angular power distribution transmitted through the refractive element as described by W. F. Williams in an article in the Microwave Journal in the July 1965 issue, pages 77 to 82. Karikomi and Kataoka, in U.S. Pat. No. 3,745,582, describe technology for steering radiated beams using a dual reflector antenna. Their graphically two-angle corrected reflectors require motion of the subreflector while keeping feed horn position fixed and the antenna is capable of steering beam angles only slightly spaced apart. No extension to offset geometry is described and aperture efficiencies are generally low and uncompensated for. In the Cassegranian antenna described by Ohm in U.S. Pat. No. 3,914,768, multiple antenna beams are formed with offset dual reflector antennas by use of a fixed main paraboloidal reflector and a hyperboloidal subreflector illuminated by a plurality of feed horns displaced transverse to the right-left symmetry plane of the antenna. In this description no means are given for scanning by feed motions, for correcting optical aberrations resulting from feed horn displacement from the focus of the hyperboloidal subreflector, nor are means suggested for improving antenna aperture efficiency, nor for reducing spillover losses.